Predictive analog to digital conversion of Doppler ultrasound signals

Analog to digital conversion in multigate Doppler ultrasound systems for blood velocity measurements is a technological challenge. The echoes must be digitized at a rate determined by the system bandwidth (typically 2 MHz), and the dynamic range is large (16 bits or more) due to the presence of strong, low-frequency Doppler clutter echoes originating from slowly moving tissue. Off-the-shelf A/D converters do not meet these requirements with the transducer configuration employed by contemporary Doppler systems. Analysis reveals a 5-b reduction in required wordlength for an A/D converter in a predictive feedback loop when the maximum clutter frequency is about 1.5% of the pulse repetition frequency. The prediction error filter is recursive. Alternatively, first- and second-order DPCM (Differential Pulse Code Modulation) yield 4 and 6 b, respectively. With short input segments (from a high-resolution Color Flow Mapper), the results are, in the above order, 4, 4, and 5 b. The results are verified by processing an experimental Doppler signal.